Whether you're doing free-building, SMP or something completely different as long as your server is running Bukkit you can get the most out of these mods and/or plugins. I have hand-picked these all myself and they all work with the latest version of Bukkit, all plugins that as of the 19th February don't work with Bukkit will not be listed.

If you havn't yet for some strange reason got Bukkit, you can acquire it HERE(direct download link). Instructions on installation etc... just Google it

Anyway, here's the list

mcMMO// To start off the list we have quite a drastic plugin, but it can really add a lot to the game. It will add a few new RPG elements to your server, such as the ability to gain skill in various new fields and repair armor and weapons.

HeroicDeath// Does it annoy you on SMP when your friend dies deep down in a cave by a monster or by falling but you don't know how? That's where this nifty plugin comes into play, however you manage to die a notification will pop-up in the chat informing the world of how you ended up with 3 arrows in your head or how your body is sprawled across 6 pieces of bedrock. However you die, this plugin will be there.

iStick// Are you a free-builder constricted by the speed of the game? Or are you just trying to de-grief some damage done by some f*cking tw*ts that joined your game with the sole purpose of being little ****s?

WorldEdit// This plugin does so much I'm going to do some shameful copy/pasting

Some of the things that WorldEdit can do include:Replace an area with different blocks and randomly distribute blocks in the areaRestore an arbitrary selected area from a backup in-gameBuild from far away by just swinging at a locationGenerate spheres, cylinders, cuboids, forests, pumpkin patches, and snowy areasCopy cuboids, paste them, rotate them, mirror them, and save them to file (MCEdit-compatible)Smooth regions, fix non-stationary water, extinguish fires, kill mobsBuild from far away, mine instantly, generate trees with a right clickDelete chunks, find out which chunks were in a region, look up your current chunkCount the statistical variation of blocks inside an areaTeleport to the location that you are looking at, ascend through ceilings, pass through wallsTie in with WorldGuard (currently not ported) to protect areasAnd much more!